Snapshot isolation in SQL / code blocking reads

Question

I suspect I don't understand fully what's going on or something weird is happening. (The first case is more likely I guess.)

The big picture:

• I'm trying to have a web-service perform certain operations asynchronously as they can be time consuming and I don't want the client to wait for the operations to finish (just query for the results every now and again to see the operation is done).
• The async code is wrapped in a transaction - in case something goes wrong, I want to be able to rollback any changes.
• Unfortunately the last step of the async code is to call a DIFFERENT service which queries the same database.
• Despite wrapping the whole thing in a Snapshot transaction, the last step fails as the service cannot read from the database.
• For that matter, while the async operation is underway I cannot perform a simple SELECT statements from the database either.

Here's a sample of the code which I'm currently using to test out the transactions (using Entity Framework 5 model first):

        using (var transaction = new System.Transactions.TransactionScope(System.Transactions.TransactionScopeOption.RequiresNew, new System.Transactions.TransactionOptions() { IsolationLevel = System.Transactions.IsolationLevel.Snapshot }))
        {
            var db = new DataModelContainer();
            Log test = new Log();
            test.Message = "TEST";
            test.Date = DateTime.UtcNow;
            test.Details = "asd";
            test.Type = "test";
            test.StackTrace = "asd";
            db.LogSet.Add(test);
            db.SaveChanges();
            using (var suppressed = new System.Transactions.TransactionScope(System.Transactions.TransactionScopeOption.Suppress))
            {
                var newDb = new DataModelContainer();
                var log = newDb.LogSet.ToArray();  //deadlock here... WHY?
            }
            test = db.LogSet.Where(l => l.Message == "TEST").Single();
            db.LogSet.Remove(test);
            db.SaveChanges();
            transaction.Complete();
        }

The code creates a simple Log entry in the database (yeah, I'm playing around at the moment so the values are rubbish). I've set the SQL database to allow snapshot isolation, and to my knowledge reads should still be permitted (these are being tested in this code by using a new, suppressed transaction and a new DataModelContainer). However, I cannot query the LogSet in the suppressed transaction or in SQL Management Studio - the whole table is locked!

So... why? Why is it locked if the transaction scope is defined as such? I've also tried other isolation levels (like ReadUncommited) and I still cannot query the table.

Could someone please provide an explanation for this behavior?

Answer 1

In SSMS, set your current isolation level to SNAPSHOT and see if that corrects your problem - it's probably set to READ COMMITTED and would therefore still block due to pending updates.

Update:

You can allow READ COMMITTED to access versioned rows DB-wide by altering the following option (and avoiding having to constantly set the current isolation level to SNAPSHOT):

ALTER DATABASE MyDatabase
SET READ_COMMITTED_SNAPSHOT ON